Relationship between foliage temperature and water stress in potatoes

Field studies were conducted in southern Idaho to evaluate the possibility of using thermal infrared measurements of potato foliage to detect soil water deficits. Concurrent measurements of foliage-air temperature differences (T_f-T_a), leaf water potential (gy_leaf) and vapor pressure deficit (VPD) were obtained from differentially-irrigated Russet Burbank and Kennebec potatoes during the 1982 and 1983 growing seasons. Foliageair temperature differences for well-watered potatoes were linearly related to VPD. Differences in T_f-T_a values between stressed and well-watered potatoes were relatively small in the early morning when evaporative demand was low. However, severe soil water deficits caused afternoon T_f-T_a values to rise as much as 8.0 C above non-stressed levels under conditions of high VPD. Foliage-air temperature differences and VPD data were used to construct a plant water stress index (PWSI) which reflected the rise in T_f-T_a above non-stressed levels at a given VPD. The PWSI was linearly related to depressions in gy_leaf caused by moderate to severe soil water deficits. However, the PWSI did not increase significantly above non-stressed values unless the soil matric potential gy_soil fell below -70 kPa (centibars). Since potatoes are normally irrigated before gy_soil falls below ?60 kPa, it appears that foliage temperature measurements cannot be used to effectively schedule irrigation for this crop.